﻿ATOMIC HEAT TO CRYSTALLINE FORM. 107 



by Mitscherlich, who from his elaborate researches deduced the well-known law, that 

 the same number of atoms combined in the same way produce the same crystalline 

 form, and that crystalline form is independent of the chemical nature of the atoms, 

 and determined by their number and relative position only. Carefully considered, 

 however, this law is seen to reveal merely the secondary or proximate cause of any 

 special crystalline form. It does not distinctly explain the phenomenon of iso- 

 morphism. Obviously in the power which aggregates the atoms, we are to look for 

 the prime cause of crystallization in general. Mitscherlich's law does not clearly 

 explain why different acids having different atomic weights and volumes, should, 

 when combined with the same base, present the same geometrical figure. According 

 to the atomic theory, the ultimate atoms composing the various chemical elements 

 differ in weight and volume. The numbers expressive of atomic weight are in some 

 measure indicative of atomic magnitude, otherwise the present great diversity in the 

 atomic numbers of the elements and their compounds could scarcely exist. And this 

 supposition receives very strong support from the well-known, but not duly appre- 

 ciated fact, that the atomic weight of a body may be very great, while its atomic 

 number is very small. Keeping in view this difference in atomic magnitude, it is 

 difficult to conceive, as indeed has been objected to Mitscherlich's law, why arsenic 

 and phosphoric acids, whose respective atomic weights are 115 and 71, should, when 

 combined with soda, produce isomorphous compounds ; why the seleniate and sulphate 

 of soda should be identical in form, when the atomic weight of selenic acid is 64, and 

 that of sulphuric acid 40, &c. 



Again, it is a well-known fact that the same body may assume two or more 

 distinct and unrelated forms. Thus, carbon presents us with regular octohedrons in 

 the diamond, and six-sided plates in graphite, or plumbago. Sulphur is also dimor- 

 phous, assuming two incompatible crystalline forms. The minerals rutile, brookite 

 and anatase are distinct crystals of titanic acid. Carbonate of lime, too, expresses 

 itself dimorphously. Such facts as these show that as far as ponderable elements are 

 concerned, crystalline form is by no means the consequence of an invariable atomic 

 constitution. Moreover, isomorphism is compatible with a considerable difference in 

 the number of atoms. Thus, according to Mitscherlich, sulphur in one of its forms 

 is isomorphous with the bisulphate of potassa. The nitrite and the nitrate of lead 

 have the same octohedral figure, yet the latter compound has two atoms more of 

 oxygen than the former. Gmelin instances a number of similar examples. 



Here, then, it remains to be explained why two or more acids having different 

 atomic weights, upon uniting with the same base, assume the same crystalline form ; 

 why the same body presents us with examples of dimorphism, trimorphism, or even, 

 as in the case of water, of polymorphism ; why some substances are isomorphous both 

 in the separate state and in combination; and lastly, why some substances replace 



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